In accordance with an increase in demand and interest for a thin film type display such as a liquid crystal display and an organic light emitting diode, recently, an effort to obtain devices having a favorable quality has rapidly grown. Silicon-based semiconductor device materials have been mainly studied as display driving devices.
Silicon has merits in terms of physical properties, life-span, and performance stability, but requires vacuum deposition and laser annealing processes to form a thin film, and thus a high-cost apparatus for the processes increases a manufacturing cost. Accordingly, recently, an effort to use a metal oxide material in a semiconductor channel layer has been conducted, and metal oxides have a possibility of a transparent device.
A thin film transistor device is basically formed of a semiconductor material, an electrode material, and a dielectric material, and recently, a study has been conducted to apply metal oxides, which is transparent in a visible ray region, to constituent elements of this representative device, thus forming a new concept device such as a transparent transistor, a transparent memory, and a transparent electrode. Inorganic zinc oxide (ZnO) as a representative transparent semiconductor material receives large attention in view of use in a channel layer of an active region in a thin film transistor due to a wide energy band gap and excellent light transmission, and zirconia (ZrO2), alumina (Al2O3), silica (SiO2), and titania (TiO2) are frequently used as an insulating layer material in the thin film transistor due to a high insulating characteristic. Further, oxides of metal such as indium or tin are transparent and have high conductivity, and thus are frequently studied as a transparent electrode material of an electrode of a transistor and a touch screen.
Despite the excellent characteristics, there are drawbacks in that a high-cost vacuum process such as sputtering and pulse laser deposition (PLD) is required to form the oxide thin film. Accordingly, a solution-based technology as a low-cost manufacturing process where mass production is feasible, that is, a method for manufacturing the thin film transistor using methods such as spin coating, inkjet printing, offset printing, and gravure printing is required. A precursor material used in the solution process should have performance suitable for each constituent element of the thin film transistor, for example, a semiconductor material should have high mobility, an excellent switching characteristic, and a high on/off ratio (Ion/off), an insulating layer should have a high insulating ability and an appropriate dielectric constant, and an electrode material should have high conductivity and low specific resistance. Additionally, if transparency is maintained while each constituent element is formed at low temperatures, an innovative method for manufacturing a transistor even on a transparent polymer substrate in addition to glass by a solution process to implement a flexible transparent transistor may be provided.
Particularly, the aforementioned required characteristics needs to be obtained under low temperature annealing at about 350° C. or less to implement a high-quality device. However, oxide thin films having excellent characteristics by low temperature annealing using a material in a solution state, and manufacturing of the thin film transistor by combining the oxide thin films very sensitively depend on a composition of a solution, and an annealing temperature and an annealing method after coating. The solution is generally constituted by a precursor (organic metal and metal salts) forming metal oxides, a solvent dissolving the precursor, a stabilizer for improving solubility (complexing agent), and other additives for controlling a coating property and viscosity. There is a difference in chemical decomposition reaction process and reaction temperature according to a type of used precursor, thereby determining crystallinity and purity of the formed oxide thin film, and a decomposition temperature and a residual amount of an organic material are determined according to a type of stabilizer and other additives. Generally, in the case of a sol-gel method and a metal-organic compound solution method (metallorganic deposition, MOD) using a metal-organic compound, metal alkoxide, and metal salts, since decomposition of the precursor and an oxide generation reaction mostly require a high temperature of about 400° C. or more, it is difficult to apply the sol-gel method and the metal-organic compound solution method to a low-temperature type flexible substrate, and in the case of a glass substrate which can be used at relatively high temperatures, since bending and separation occur, misalignment and cracks are formed during a subsequent process. Accordingly, currently, annealing is performed in advance before a semiconductor process through a pre-compaction process, but there are limitations in that this process is an additional process and is difficult to be applied to a large-sized glass substrate.
A typical conventional method for manufacturing an oxide semiconductor is a vacuum deposition, and currently, there are many reports regarding a sol-gel method, a colloidal particle method, and an organometallic deposition method, but these methods require annealing and post-treatment processes at about 400° C. or more.
Meanwhile, in the case of the oxide semiconductor, transition metal added to the oxide semiconductor may be reduced to increase a carrier concentration and oxygen defects in a thin film. Accordingly, transition metal serves to increase conductivity of the thin film to become an alternative for overcoming a limitation of mobility, but when positive bias stress is continuously applied, that is, when a positive gate voltage is continuously applied or a voltage is repeatedly applied to perform driving, there is a limitation in that a threshold voltage of the device is significantly changed in a positive direction. Further, in the case of a typical display such as LCD and OLED, when a negative gate voltage is continuously applied in the device in an off state, heat is generated, and thus there is a limitation in that the threshold voltage is changed in a negative direction.
This unstable characteristic causes a very large limitation in views of stability of the transistor performing a switching role, and thus the typical display has physical properties that are difficult to be applied to devices in practice.